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Neural Tract Abnormalities Reported in Imaging Study on Writer's Cramp

Sophisticated imaging of the brains of people with writer's cramp revealed abnormalities in neural pathways that connect the primary sensorimotor areas with subcortical structures, according to a study in the April edition of the Archives of Neurology.

The work by a team of French researchers does not offer a definite explanation for what causes writer's cramp, a task-specific and often debilitating form of primary dystonia. But it does help further understanding of a baffling disorder, which can render people unable to write with a pencil or pen because of muscle spasms, involuntary contractions, and stiffness in the hands.

The brain of a patient with writer's cramp appears normal using standard MRI, but advances in imaging and analysis techniques have allowed researchers to identify subtle structural differences.

The French team was able to trace fiber tracts using diffusion-tensor magnetic resonance imaging (DTI), which assesses white matter integrity, and a measurement known as fractional anisotropy (FA).

The investigators compared 26 right-handed people between the ages of 21 and 65 who had task-specific dystonia triggered by writing but no impairment in other motor tasks with 26 healthy right-handed controls of similar age and sex. None of the patients with writer's cramp had received botulinum injections in the previous six months.

An analysis of the scans from the two groups revealed that “fractional anisotropy was significantly increased in patients with writer's cramp than in controls, when measured in the white matter of the posterior limb of the internal capsule, and the adjacent thalamus,” Dr. Delmaire said.

“The study suggests that writer's cramp is associated with microstructural changes involving fibers that carry afferents and efferents to and from the primary sensorimotor cortex,' the researchers concluded, noting that the findings “are in accordance with previous studies” involving focal dystonia. “However, it is unknown how these changes relate to the physiopathology of the disease.”

“Diffusion-tensor imaging revealed increased fractional anisotropy bilaterally in the white matter of the posterior limb of the internal capsule and adjacent structures in the patients with writer's cramp,” the team reported. “Fiber tracking demonstrated that fractional anisotropy changes involve fiber tracts connecting the primary sensorimotor areas with subcortical structures.”

MORE WORK IS NEEDED

First author Christine Delmaire, MD, of the Center for NeuroImaging Research of the Groupe Hospitalier Pitié-Salpêtrière in Paris, told Neurology Today that the results serve to “strengthen the role of sensorimotor structures and their connections in the pathophysiology of the disorder. But she and other researchers who study writer's cramp said additional research is needed to determine what is going wrong in the brain.

“This is a nice study and we need to do a whole series of these before we get some general understanding or where the problem is and what we can do for it,” Mark Hallett, MD, head of the Human Motor Control Section at the NINDS, told Neurology Today. He said that most of the imaging research on writer's cramp has focused on the grey matter, while the current study focused on detecting changes in the white matter. His research on writer's cramp has pointed to problems in the sensorimotor cortex.

“It is unlikely the primary abnormality is of the white matter itself,” he said, noting that the white matter abnormalities identified by this latest study “are probably secondary to other changes.”

He noted that writer's cramp is the third most common form of focal dystonia, behind cervical dystonia and blepharospasm.

Steven Frucht, MD, associate professor of clinical neurology at Columbia University who is an expert on movement disorders, told Neurology Today that he was impressed by the sophisticated scanning approach used by the French team, which he said provided an “anatomical measure of circuitry.”

“This is really an outstanding study in every way,” he said. He was particularly interested in the finding that bilateral abnormalities were identified in the brains of people with writer's cramp, given that the disorder usually involves only one hand.

“There are bilateral abnormalities, so how does that make sense?” he said. “The only way it makes sense is if these abnormalities are due to an underlying genetic trait that people are born with.”

GENETIC, ENVIRONMENTAL FACTORS

Most researchers believe that people with writer's cramp have a genetic profile that predisposes them to the condition, but that environmental factors influence whether they develop problems later in life. The disorder often surfaces after prolonged writing.

Dr. Hallett said writer's cramp has been reported to be a particular problem in India, where the education system places a strong emphasis on writing and neat penmanship.

“The cause is probably multifactorial,” likely involving multiple genes and environmental triggers, said Dr. Hallett. Many people complain that they have “writer's cramp,” but the diagnosable disorder differs from the fatigue and temporary pain that people may experience from an extended period of writing. The usual treatment for writer's cramp includes injections of botulinum toxin.

Dr. Delmaire said she and her colleagues are continuing to work on the problem, using functional MRI to study motor learning processes in the brain along with magnetic resonance spectroscopy to study metabolic changes.

REFERENCES

• Delmaire C, Vidailhet M, Lehéricy S, et al. Diffusion abnormalities in the primary sensorimotor pathways in writer's cramp. Arch Neurol 2009;66(4):502–508